Directive antenna



June 10, 1947. G. H. BROWN ET AL DIRECTIVE ANTENNA Filed Jan. 22, 1944 2Sheets-Sheet l Patented June 10, 1947 DIRECTIVE ANTENNA George H. Brownand Oakley M. Woodward, Jr., Princeton, N. J assignors to RadioCorporation of America, a corporation of Delaware Application January22, 1944, Serial No. 519,266

2 Claims.

This invention relates to directive antenna systems, and moreparticularly to the art of radiating and/or receiving energy directivelythroughout a broad band of frequencies with a single antenna system.

The principal object of th instant invention is to provide an improvedmethod of and means for receiving directively and/or transmittingdirectively radio frequency energy.

Another object of the invention is to provide an improved directiveantenna system of thetype comprising a single radiator and a parabolicreflector, which is capable of efficient operation throughout a widefrequency band.

A further object is to provide an antenna system of the described typewhich i imple in design and construction, and may be embodiedpractically in a structure which is mechanically strong, yet relativelylight and inexpensive.

These and other objects will become apparent to those skilled in the artupon consideration of the following description with reference to theaccompanying drawing, of which Figure 1 is a schematic perspectivediagram of an antenna system according to the present invention, Figure2 is a sectional elevation of a broad-band radiator element employed inthe system of Figure 1, Figure 3 is a graph illustrating variations inthe standing wave ratio on a transmission line connected to the systemof Figure 1 as a function of frequency, Figure 4 is a graph illustratingthe directivity of the system of Figure 1 as a function of frequency,Figure 5 is a graph illustrating variations with frequency in the powerreceived by the antenna of Figure 1, and Figure 6 is a graph of therelationship between standing wave ratio and received power.

It is Well known in prior art practice to employ parabolic reflectors'for providing antenna directivity. A discussion of parabolic reflectorswill be found on pages 837-841 of Radio Engineers Handbook by F. E.Terman, published by Mo- Graw-Hill Book Co., 1943. Such reflectors areused, particularly at very high frequencies, to rovide relatively sharpdirective characteristics. Ordinarily the dimensions of the reflector,and its focal length, are large relative to the wavelength of the energyto be transmitted, and so related that the focus lines in or near theplane of the mouth of the reflector. With a reflector which is a largenumber of Wavelengths in width at its mouth, the beam formed byreflected energy will be very narrow and very concentrated, and thedirective pattern will be substantially unaffected by the unreflectedenergy radiated in a diffuse manner forward from the antenna. Atsomewhat lower frequencies, it is impractical to use a parabola with amouth opening very many wavelengths across, because of the prohibitivesize of the reflector. In the practice of the present invention, arelatively small parabolic reflector is used, with a broad-band radiatorelement. The field contributed by the non-reflected direct radiation isof the same order of magnitude as that produced by the energy reflectedfrom the parabola. These fields are combined so as to produce aresultant field pattern which is relatively free of secondary lobes overa wide range of frequency, while the impedance presented by the radiatorremains relatively constant.

' Refer to Figure 1. A reflector l, of cylindrical parabolic form, iscovered at one end by a conductive bottom sheet or screen 3. A radiatorelement 5 is supported on the screen 3, relatively close to the surfacethereof, and is positioned coaxially with the focal line I of thereflector. The reflector I comprises a framework of pipe or tubing towhich the bottom screen 3 and upright surface members 9 and II arewelded or otherwise secured. The bottom screen 3 and the rear surface 9may be constructed of perforated sheet metal. The outer portions ll ofthe reflector are formed by spaced parallel wires secured at their endsto the tubular frame members. It is to be understood that either solidor perforated sheet material could be used throughout, al-

though in the interests of lightness and strength the describedconstruction is preferred.

Referring to Figure 2, the radiator element 5 comprises a hollowcylindrical body t3 of brass or other conductive material, with top andbottom end walls [5 and I1 soldered or similarly secured thereto. Thebottom end I! is secured to a fitting 19 which engages and iselectrically connected to the inner conductor 2| of a coaxial couplingmember 23. The coupling 23 is adapted to be engaged at its lower end bya complementary coupling device at the end of a coaxial cable (notshown). The outer conductor 25 of the coupling 23 extends through and issecured to a plate 21, which is adapted to be secured to the bottomscreen 3 of the reflector assembly.

If the above-described combination wereto be used at a single frequency,with a parabolic reflector of relatively small mouth width W (seeFigure 1) of, for example, two Wavelengths, the focal length F would bemade an odd number of quarter wavelengths so that energy reflected fromthe parabola would add, along the axis of the beam, to that radiateddirectly from the antenna. At

other frequencies, particularly those at which the focal distance F isan even number of quarter wavelengths, the direct energy will subtractfrom the reflected energy along the beam axis, diminishing the mainpattern lobe and producing relatively large side lobes. We have foundthat by making the focal distance F a single quarter wavelength, ratherthan a large number of quarter wavelengths, the frequency may be variedover a total range of approximately two to o e without producingseriously large secondary pattern lobes. larger mouth width (measured inwavelengths) at higher frequencies than at lower frequencies the rangeof effective operation may be extended much further toward the higherfrequencies than toward the lower frequencies. tem designed to operateover the range 350 to 725 megacycles, the optimum focal distance wasfound to be seven inches, or one quarter wavelength at 423 megacycles.The focal length is 0.208 wavelength at 350 megacycles, and 0.43wavelength at 725 megacycles.

Broad-band radiator elements of the type having relatively large surfaceareas in proportion to their lengths are well known, such elements beingused in television antennas and other applications where broad-bandradiation is required. We have discovered that in the system shown inFigure 1, a cylindrical radiator having approximately equal dimensionsof length and diameter, of approximately one quarter wavelength at theupper frequency limit, will provide the minimum variation in impedanceover the frequency band. For the band of 350 to 725 megacycles, the bestvalues for the dimensions D and L (Figure 2) are 4%. inches and 4 inchesrespectively. Figure 3 illustrates the variations of impedance withvariation in frequency, in terms of the standing wave ratio measured ona transmission line connected between the radiator element and a sourceof radio frequency power.

The performance of the described antenna system when used for receptionis illustrated by the curve of Figure 5. Designating as P1 the powerreceived under operating conditions, with constant field strength, andas P2 the power which would be received if a perfect impedance matchexisted between the antenna and a, receiver connected to it, the ratioPl/PZ varies with frequency as shown by Figure 5. The curve of Figure isrelated to that of Figure 3 as follows:

where R is the standing wave ratio. This relationship is shown by thecurve of Figure 6.

Figure 4 illustrates the width of the beam provided by the system ofFigure 1, as a function of frequency. The curve of Figure 4 was obtainedwith a reflector having a mouth width of 87 inches, about 2.4wavelengths at 350 megacycles. Owing to reflection from the bottomscreen, the axis of the beam is tilted upward. If a horizontal beam isdesired, the assembly is tilted forward from the vertical axis through acorresponding Thus for a sys- However, since the parabola has a angleor, as illustrated in Figure 1. This angle is approximately 20 degrees,with a reflector of the dimensions described herein.

The invention has been described as a broadband directional antennasystem, comprising a relatively small parabolic reflector and a short,large diameter cylindrical radiator. The focal length of the parabola isapproximately one fifth wavelength at the low frequency end of the band.The radiator element has a diameter approximately equal to its length,which is approximately one eighth wavelength at the low frequency end ofthe band. With the described dimensions, which are given by way ofexample rather than by way of limitation, the system will provideeflicient directive operation up to a frequency of approximately twicethe low frequency limit.

We claim as our invention:

1. A directive antenna system including a reflector of cylindricalparabolic shape, with a focal length substantially equal to one fifthwavelength at the lower limit of the band of frequencies over which thesystem is to operate and a mouth width of approximately two wavelengthsat the lowest frequency at which the system is to operate, and abroad-band radiator element positioned substantially in the focal lineof said refiector, said radiator element comprising a cylindricalconductive member of length and diameter approximately equal to eachother and approximately equal to one eighth'wavelength at the lowestfrequency at which the system is to operate.

2. A directive antenna system including a reflector of cylindricalparabolic shape, with a focal length substantially equal to one fifthwavelength at the lower limit of the band of frequencies over which thesystem is to operate and a mouth width REFERENGES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,275,646 Peterson Mar. 10, 19421,990,649 Ilberg Feb. 12, 1935 OTHER REFERENCES Radio EngineersHandbook, by F. E. Terman, first ed., 1943. Published by McGraw-HillBook Co., New York, N. Y., pages 838 and 839.

